1. Field of the Invention
This invention relates to a high-resolution position sensitive radiation detector, which can also detect the position in depth of radiation absorption in a scintillator.
2. Prior Art
A conventional position-sensitive radiation detector has been disclosed in which independent scintillators in a columnar form are bound together in a mosaic and connected to a position-sensitive photodetector (Japanese Patent Application Laid-open No. 237081/1986, Japanese Patent Application Laid-open No. 271486/1986). Another device is known in which a monolithic plate scintillator is connected to a position-sensitive photo-detector such as a photomultiplier tube (PMT) array, or the like for use as a position-sensitive radiation detector which can also detect the position in depth of radiation absorption in a scintillator (J. G. Rogers et al., Phys. Med. Biol. (1986), Vol. 31, No. 10, pp. 1061 to 1090).
FIG. 1 shows a conventional radiation detector in which a position-sensitive photo-detector is connected to a plate-like scintillator. In this drawing, reference numeral 1 represents the scintillator, reference numeral 2 represents the position-sensitive photo-detector, reference numerals 3 and 4 represent gamma rays, and reference numerals 5 and 6 represent scintillation light.
In this figure, the positions upon which the gamma rays 3 and 4 are made incident are detected by making use of the distribution positions of scintillation light outputted from the scintillator 1, for example, by calculating the center of gravity of the outputted light distribution. The radiation absorbed positions A and B in a vertical direction (the depth of scintillation points) are also detected by using the extent of distribution of the outputted light.
However, the type of position-sensitive radiation detector which uses a mosaic-shaped BGO (Bismuth Germanate) cannot obtain the information on the depth of the radiation absorption in a scintillator, that is, the information on the depth of the scintillation point. Furthermore, assembling such a detector requires much labor and is costly. Although the radiation detector device shown in FIG. 1 can obtain the information on the depth of the radiation absorption in a scintillator, the light is distributed over a very large area, particularly when the scintillation light is emitted in an upper portion of the scintillator. This causes the deterioration of position resolution. The position resolution R is given by R .alpha..sigma./.sqroot.N, where the spread of light distribution is represented by .sigma., and the number of emitted photons is represented by N. Therefore, in the detector shown in FIG. 1 in which the spread of light output distribution .sigma. is large, the position resolution R deteriorates.
To obtain the information on the depth of the light scintillation point in the scintillator it is helpful to reduce to positional error called a parallax error, which is important for applications to positron CT and so on. The occurrence of the parallax error .DELTA. is illustrated in FIG. 2. When two radiation rays are diagonally made incident on the scintillator 1 with the same incident direction and position, are absorbed at the different positions A and B, and scintillation light is emitted from positions A and B, these two radation rays are detected as if these two rays have different incident positions (the parallax error .DELTA.). (If the scintillator 1 is sufficiently thin, that is, a width t is small enough, these two radiation rays are substantially detected as the same ones with the same incident position O.) If the information D on the absorption depth is obtianed, this type of error can be eliminated.